Computational analysis of apatite-type compounds for band gap engineering: DFT calculations and structure prediction using tetrahedral substitution

Mineral apatite compounds have attracted significant interest due to their chemical stability and adjustable hexagonal structure, which makes them suitable as new photovoltaic functional materials. The band gap of natural apatite is ~ 5.45 eV, and such a large value limits their applications in the field of catalysis and energy devices. In this research, we designed a method to narrow the band gap via the tetrahedral substitution effect in apatite-based compounds. The density functional theory (DFT) and experimental investigation of the electronic and optical properties revealed that the continuous incorporation of [MO4]4– tetrahedrons (M = Si, Ge, Sn, and Mn) into the crystal lattice can significantly reduce the band gap. In particular, this phenomenon was observed when the [MnO4]4– tetrahedron replaces the [PO4]4– tetrahedron because of the formation of a Mn 3d-derived conduction band minimum (CBM) and interacts with other elements, leading to band broadening and obvious reduction of the band gap. This approach allowed us to propose a novel scheme in the band gap engineering of apatite-based compounds toward an entire spectral range modification.